1. Field of the Invention
This invention relates generally to power distribution systems and more particularly, to a method and apparatus for a circuit protection system providing dynamic delay times for circuit breakers throughout the system.
2. Description of the Prior Art
In power distribution systems, power is distributed to various loads and is typically divided into branch circuits, which supply power to specified loads. The branch circuits also can be connected to various other power distribution equipment, such as, transformers that step down the supply voltage for use by a specific piece of electrical equipment.
Due to the concern of an abnormal power condition in the system, i.e., a fault, it is known to provide circuit protective devices, e.g., circuit breakers to protect the various loads, as well as the power distribution equipment. The circuit breakers seek to prevent or minimize damage and typically function automatically. The circuit breakers also seek to minimize the extent and duration of electrical service interruption in the event of a fault.
It is further known to utilize upstream circuit breakers having pre-programmed time delays so that the downstream circuit breakers are provided with an opportunity to clear the fault before the upstream circuit breaker opens or trips. In a known zone selective interlock system, a downstream circuit breaker can be in direct communication with an upstream circuit breaker through wiring such that the downstream circuit breaker sends a signal to the upstream circuit breaker placing the upstream circuit breaker in a restrained mode. In the restrained mode, the circuit breaker temporarily restrains from opening or tripping until after a pre-determined time delay has timed out. The circuit breakers each have pre-programmed time delay settings incorporated therein. This type of system provides for time delays based upon pre-set, invariable time periods associated with the upstream circuit breaker. Thus, the upstream circuit breaker will delay tripping by a pre-set period of time regardless of the location of the fault in the power distribution system.
The circuit breakers, can be arranged in a hierarchy or tree configuration having a plurality of layers or levels with the upstream circuit breakers closer to the power source and the downstream circuit breakers closer to the loads. In order to minimize service interruption, the circuit breaker nearest the fault will first attempt to interrupt the fault current. If this first circuit breaker does not timely clear the fault, then the next upstream circuit breaker will attempt to do so. However, this can result in the problem of a circuit breaker multiple levels upstream from a fault being tripped when the fault is detected, which causes power loss to the multiple levels of loads downstream that should otherwise be unaffected.
Such a system does not delay the upstream circuit breakers based upon an optimal time period to provide the downstream circuit breaker with the opportunity to clear the fault. Where a circuit has downstream circuit branches and circuit breakers having differing temporal properties, such as, for example, clearing time or pre-set delay time of the circuit breakers, such a system fails to account for these differences. This increases the risk of damage to the system where an upstream circuit breaker has a pre-set time delay that is too long based on the location of the fault. This also decreases the efficiency of the system where an upstream circuit breaker has a pre-set time delay that is too short based on the location of the fault and opens before the downstream circuit breaker has a full opportunity to clear the fault. This system also suffers from the drawback of the need to hardwire the upstream circuit breakers with each of the downstream circuit breakers. In a multi-tiered and multi-source system, this can require a complex and costly wiring scheme.
Accordingly, there is a need for circuit protection systems incorporated into power distribution systems that decrease the risk of damage and increase efficiency of the power distribution system. There is a further need for protection systems that can vary the zones of protection and the time delays of protection as the power distribution system changes and provide optimized protection without sacrificing selectivity.